1. Field of the Invention
The instant invention is in the field of photochromic compositions. It is more particularly directed to particulate photochromic materials comprising organic photochromic pigments coated with inorganic materials.
2. Prior Art
It is known to combine a photochromic material and a resinous material, for instance, U.S. Pat. No. 3,761,422 relates to the preparation of a photochromic plastisol composition containing metal complexes of diphenylthiocarbazone which can be formed into a film. U.S. Pat. No. 3,565,814 relates to a photochromic composition having a fast rate of color change comprising a polymer of lauryl methacrylate having dispersed throughout the body thereof various benzospiropyran compounds. U.S. Pat. No. 3,666,352 relates to a photochromic lens comprising a sheet of vinyl chloridevinyl acetate copolymer containing a mercury dithiozonate compound laminated between glass or plastic layers.
The need for protection of photochromic materials against atmospheric oxygen has also been recognized by the prior art as a means of insuring fast action and longer reversibility, for instance, U.S. Pat. No. 3,716,489 discloses a method of producing a fast-acting photochromic filter in which the photochromic material is in the form of a solid solution in a solid optically transparent epoxy polymer or alternately a polycarbonate polymer matrix.
In British Pat. No. 1,191,483 there is disclosed that a chromotropic dye can be sorbed into silica gel. Diphenyl thiocarbazones can be sorbed first and then reacted with desired metal cations to produce a useful chromotropic colorant composition or the metal salt absorbed directly.